Tri Bike Fit Calculator
Estimate triathlon aero position geometry from body measurements including saddle height, aerobar reach, frame size, and crank length.
About this calculator
Triathlon bike fit differs from road fit in one key way: because you have to run immediately after riding, the position trades some pedaling comfort for a more forward hip angle that leaves your running muscles fresher. This calculator's saddle height (inseam × 0.885) sits just below the classic LeMond road-fit ratio of 0.887, reflecting that slightly lower tri convention. Saddle setback is minimal — inseam × 0.02, capped at zero minimum — versus the 5-7cm typical of road bikes, because sliding the saddle forward over the bottom bracket rotates the pelvis into a more sustainable aero position and shortens the effective reach to the aerobars.
Aerobar reach comes from torso length alone (torso × 0.95), while stack — how high the pads sit above the bottom bracket — is your saddle height plus a saddle-to-pad drop that the calculator estimates from your torso-to-height ratio as a rough proxy for flexibility: a longer torso relative to height allows a more aggressive (deeper) drop. Crank length recommends slightly shorter than the standard road formula (inseam × 2.16, minus 2.5mm) because a shorter crank arm reduces the maximum hip flexion angle at the top of the pedal stroke, which matters more when you're already folded into an aero position. All of these are geometric starting points derived from body measurements alone — they don't account for flexibility, injury history, or bike-specific stack/reach limitations, so treat the output as a fitting session's opening bid, not its final answer.
Inputs
Results
Saddle height (cm)
72.6
How to Use This Calculator
- Measure and enter your Height, Inseam, Torso Length, and Arm Length in centimeters.
- Review the recommended Saddle Height (BB to top) as your starting point.
- Check Frame Size, Effective Top Tube, Aerobar Reach, and Aerobar Pad Width.
- Use Saddle-Pad Drop and Hip Angle to assess your aero position comfort and power output.
- Cross-reference these values with a professional bike fitter for final dialing.
How the result changes with Inseam (cm)
| Inseam (cm) | Saddle height (cm) |
|---|---|
| 60 | 53.1 |
| 62 | 54.9 |
| 105 | 92.9 |
What each input means
- Height (cm)
- Your total standing height in centimeters.
- Inseam (cm)
- Inside leg measurement from floor to crotch (stand barefoot against a wall).
- Torso length (cm)
- Distance from the top of your hip bone (iliac crest) to the sternal notch at the base of your neck.
- Arm length (cm)
- Measured from the acromion (shoulder point) to the center of the wrist with arm extended.
What each result means
- Saddle height (cm)
- Center of bottom bracket to top of saddle. Based on 88.5% of inseam for tri position.
- Frame size (cm)
- Recommended seat tube length (center-to-top) for a triathlon/TT frame.
- Effective top tube (cm)
- Horizontal distance from head tube center to seat post center.
- Saddle setback (cm)
- Saddle position behind the bottom bracket. Tri bikes use minimal setback (0-3cm).
- Aerobar reach (cm)
- Distance from saddle nose to aerobar pads.
- Pad width (cm)
- Distance between aerobar pad centers. Narrower is more aerodynamic.
- Saddle-pad drop (cm)
- Height difference between saddle top and aerobar pads. Negative means pads are lower.
- Aerobar stack (cm)
- Height of aerobar pads above bottom bracket center.
- Crank length (mm)
- Recommended crank arm length. Tri bikes often use slightly shorter cranks.
- KOPS forward (cm)
- Knee-over-pedal-spindle: how far forward of the pedal spindle your knee is at 3 o'clock.
- Hip angle (°)
- Estimated hip angle in aero position. Tri bikes target 75-85° for sustainable power.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersHeight (cm) = 175, Inseam (cm) = 82, Torso length (cm) = 58, Arm length (cm) = 63 = 4 input(s) provided
- Calculate Saddle heightSaddle height = inseamCm * 0.88572.6 = 72.6
- Calculate Frame sizeFrame size = inseamCm * 0.6553.3 = 53.3
- Calculate Effective top tubeEffective top tube = (torsoLengthCm + armLengthCm) * 0.4858.1 = 58.1
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why is my recommended saddle height lower than what I've read for road cycling fit?
This calculator uses inseam × 0.885 for triathlon position, deliberately just below the classic LeMond road-fit ratio of about 0.887. The difference is small but intentional — triathlon's more forward, aero-focused position pairs with a very slightly lower saddle than road cycling's more upright, power-focused setup.
Why does the calculator recommend almost no saddle setback compared to a road bike?
Saddle setback is calculated as inseam × 0.02, capped at a zero minimum, versus the 5-7cm typical for road bikes. Sliding the saddle forward over the bottom bracket rotates your pelvis into a more sustainable aero position and shortens the reach needed to the aerobars — a deliberate tradeoff of pure pedaling efficiency for a position you can hold in the aero bars for hours.
How does the calculator estimate my saddle-to-pad drop, and why might it be wrong for me?
It uses your torso length divided by height as a rough proxy for flexibility: a ratio above 0.34 gets a -7cm drop, above 0.32 gets -5cm, and anything lower gets -3cm. This is only an approximation — actual flexibility, core strength, and lower-back tolerance vary a lot between people with the same torso-to-height ratio, so a fitter should adjust it based on how you actually feel and perform in the position.
Why does the recommended crank length come out shorter than the standard road-bike formula?
The calculator takes the standard formula, inseam × 2.16, and subtracts 2.5mm specifically for triathlon. A shorter crank arm reduces the maximum hip flexion angle at the top of the pedal stroke, which matters more in an already-folded aero position — it helps you avoid closing your hip angle too far and losing power or comfort when you're already crouched forward.
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